Geotechnical Evaluation of a Small Levee System for FEMA Certification

Rebecca Money, PE, GE, CFM

Purpose

Provide an overview and an understanding for Cities/Counties/Districts of what it takes from a Geotechnical Engineering perspective to prepare a levee system for certification.

Introduction

Client: Sacramento Area Flood Control Agency (SAFCA)North Area Streams Study Area located just north of Sacramento, CaliforniaIncluded five levee segments:

Dry Creek right and left bank levees (3.8 miles)Arcade Creek right and left bank levees (4.2 miles)Natomas East Main Drainage Canal (NEMDC) east bank levee (3.9 miles)

Site Location

Background

SAFCA tasked with certifying these levees by 2020Kleinfelder reviewed multiple levee systems to identify areas of concern

Background – Phase 1

Phase 1:Data collection and Geotechnical Data Report

Phase 2:Engineering Evaluation and Problem Identification Report

Phase 3:Final Design and/or Levee Certification

Introduction

Levee: Dry Creek right and left bank leveesLength: Left bank levee 2.2 miles, right bank levee 1.6 milesWaterways: Dry Creek and Robla Creek

Phase 1

Existing Information

Explorations: NAS Total145 borings49 CPTs

Explorations: Dry Creek Only47 borings18 CPTsDepths were mostly shallow

Not Sufficient (depth and spacing)

Existing Information

Sampling and Laboratory TestingMost borings had 5’ sample intervalsLab testing included:

Index and strength testing

Not Sufficient (quantity and depth)

Existing Information

Geologic MappingRight leveeChannels,Modesto and Riverbank Formations

Left leveeChannels,Vernal Pools,Modesto and Riverbank Formations

(Fugro WLA)

Existing Information

Topographic Maps:LiDAR Site specific ground surveys

(Fugro West)

Existing Information

HEM Survey: Geophysical survey that evaluates changes resistivity in subsurface conditions. Typically shows differences between high resistivity (sands) and low resistivity (clays)

(Fugro Airborne Services)

Existing Information

As-Built Drawings: 1950’s USACE Construction1995 and 1997 ConstructionFinal Construction ReportsCommunication with field Geotechnical Engineer

Existing Information

Construction Inspection ReportsGeotechnical inspector was available for consultationFinal construction reports detailing key trench depths

Sufficient and Invaluable

Existing Information

Past Performance:Historic water levelsFlood patrol notesNewspaper articles

Historical Water Levels

Historical High Water Marks

New Information

Levee InspectionBorings

6 borings Lab

Moisture content, unit weight, Atterberg Limits, grain size analysis

Phase 2

Reach Selection

Levee Segments divided into reaches based on:

Subsurface conditionsPast performanceConstruction/Remediation historyGeomorphology changesTopographic featuresConstruction features

Reach Selection

Reach Limits

Analysis Cross Section

Critical analysis cross section:1 section per reachBased on:

TopographyStratigraphyPast performance

Representative for entire reach – Base modelSensitivity analysis performed if needed

Material Property Selection

Analysis

Engineering analysis performed to evaluate:

Levee GeometryThrough SeepageUnderseepageWaterside and Landside Slope StabilitySettlementSeismic Evaluation

Using 100-year WSE (1-percent chance flood)

Analysis Methods

USACE guidelines were used for evaluation of seepage, stability, and settlement (EM 1110-2-1913, ETL 1110-2-569, and SOP SPK EDG-03)USACE process for National Flood Insurance Program (NFIP) guidance for levee system evaluation (EC 1110-2-6067)Code of Federal Regulations (44 CFR 65.10)DWR Urban Levee Design Criteria (ULDC) used for seismic

Analysis CriteriaThrough seepage: water exiting the landside levee slope (daylight of phreatic surface) and presence of erodible soilsUnderseepage: Exit gradient less than 0.5 at landside levee toeStability: Minimum Factor of Safety of 1.1 for Case II – Sudden Drawdown and 1.4 for Case III -Steady-State Seepage ConditionSettlement: N/A –Proposed project does not involve new construction or modification to levee prism.

Analysis Results –Through Seepage

Layer 3, construction to raise levee and reconstruct waterside/landside slope

Watch for daylight of phreatic surface on landside levee slope

Analysis Results –Underseepage

Gradients calculated at the landside levee toe and at the bottom of any lower topography (i.e. ditches)Keyways from construction tip into hardpan

Analysis Results –Landside Stability

Landside slopes typically 2H:1V

Analysis Results –Waterside Stability

Waterside slopes typically 2.5H:1V to 4H:1V

Analysis Results –Seismic Deformation

Qualitative evaluation to estimate liquefaction potential of subsurface soilsDeformation analysis performed only if soils are liquefiable under design ground motions7 borings have potential liquefiable soils in the upper 10 feetAll were thin, discontinuous layersTherefore, no deformation analysis was performed

Conclusions

Geometry – meets criteriaThrough Seepage – meets criteriaUnderseepage – met criteria in all but 1 location which was further explored in Phase 3 and determined to meet criteriaLandside stability – meets criteriaWaterside stability – meets criteriaSeismic – additional evaluation should be performed during design based on results of qualitative evaluation

Conclusions

Phase 3 –One outstanding area needed additional review for Dry Creek left bank levee.Additional exploration and revision to analysis confirmed it met criteria

Ready for Certification

Levee Certification

Dry Creek right and left bank levees meet USACE levee criteriaRegistered Professional EngineerPreparation of Engineers Opinion for inclusion in Levee Accreditation Package Appendices for:

Embankment and Foundation StabilitySettlement PotentialOther Civil Components also included

Thank you!

Geotechnical Evaluation of a Small Levee System for FEMA CertificationPurposeIntroductionSite LocationBackgroundBackground – Phase 1IntroductionPhase 1Existing InformationExisting InformationExisting InformationExisting InformationExisting InformationExisting InformationExisting InformationExisting InformationHistorical Water LevelsHistorical High Water MarksNew InformationPhase 2Reach SelectionReach SelectionReach LimitsAnalysis Cross SectionSlide Number 25Material Property SelectionAnalysisAnalysis MethodsAnalysis CriteriaAnalysis Results – �Through SeepageAnalysis Results – �UnderseepageAnalysis Results – �Landside StabilityAnalysis Results – �Waterside StabilityAnalysis Results – �Seismic DeformationConclusionsConclusionsLevee CertificationThank you!